Isocitrate dehydrogenases (IDHs) are enzymes that participate in the citric acid cycle (cellular metabolism). They catalyze the oxidative decarboxylation of isocitrate to 2-oxoglutarate (i.e., α-ketoglutarate, α-KG). There are three isoforms within the IDH family. IDH-1, expressed in the cytoplasm and peroxisome, IDH-2, localized in the mitochondria, both utilize NADP+ as the cofactor and exist as homodimers. IDH-3 is localized in mitochondrial matrix and utilizes NAD+ as a cofactor and exists as tetramer. Mutations in IDH-1 (cytosolic) and IDH-2 (mitochondrial) have been identified in various diseases or disorders including glioma, glioblastoma multiforme, paraganglioma, supratentorial primordial neuroectodermal tumors, acute myeloid leukemia (AML), prostate cancer, thyroid cancer, colon cancer, chondrosarcoma, cholangiocarcinoma, peripheral T-cell lymphoma, and melanoma (L. Deng et al., Trends Mol. Med., 2010, 16, 387; T. Shibata et al., Am. J. Pathol., 2011, 178 (3), 1395; Gaal et al., J. Clin. Endocrinol. Metab. 2010; Hayden et al., Cell Cycle, 2009; Balss et al., Acta Neuropathol., 2008). The mutations have been found at or near key residues in the active site: G97D, R100, R132, H133Q, and A134D for IDH1, and R140 and R172 for IDH2. (See L. Deng et al., Nature, 2009, 462, 739; L. Sellner et al., Eur. J. Haematol., 2011, 85, 457).
Mutant forms of IDH-1 and IDH-2 have been shown to lose wild type activity, and instead exhibit a neomorphic activity (also known as a gain of function activity), of reducing alpha-ketoglutarate to 2-hydroxyglutarate (2-HG). (See P. S. Ward et al., Cancer Cell, 2010, 17, 225; Zhao et. al., Science 324, 261 (2009); Dang et. al Nature 462, 739 (2009)). In general, production of 2-HG is enantiospecific, resulting in generation of the D-enantiomer (also known as the R enantiomer or R-2-HG). Normal cells have low basal levels of 2-HG, whereas cells harboring mutations in IDH1 or IDH2 show significantly elevated levels of 2-HG. High levels of 2-HG have also been detected in tumors harboring the mutations. For example, high levels of 2-HG have been detected in the plasma of patients with mutant IDH containing AML. (See S. Gross et al., J. Exp. Med., 2010, 207 (2), 339). High levels of 2-HG have been shown to block α-KG dependent DNA and histone demethylases, and ultimately to result in improper dedifferentiation of hematopoietic progenitor cells in AML patients (Wang et. al., Science 340, 622 (2013); Losman et al., Science 339, 1621 (2013)).
Furthermore, patients with Oilier Disease and Mafucci Syndrome (two rare disorders that predispose to cartilaginous tumors) have been shown to be somatically mosaic for IDH1 and 2 mutations and exhibit high levels of D-2-HG. (See Amary et al., Nature Genetics, 2011 and Pansuriya et al., Nature Genetics, 2011).
The inhibition of mt-IDHs and their neomorphic activity with small molecule inhibitors therefore has the potential to be a treatment for cancers and other disorders of cellular proliferation.